1. Field of the Invention
The present invention relates generally to airfoil performance augmentation through ducting and flow of pressurized air or gas over the surface of an airfoil, and more specifically pertains to a method and structure for selectively directing such flow either rearward to produce thrust and augment lift, or forward to provide reverse-thrust braking and spoiling of the aerodynamic lift of the airfoil.
2. Description of the Prior Art
Short take-off or landing aircraft (STOL) require, in take-off mode, a highly efficient lift producing airfoil operable with relatively low air speed. Conversely, upon landing, it is desirable that the airfoil be inefficient to transfer more of the aircraft weight to the supporting landing gear and thus enhance the effectiveness of the brakes.
Also, when utilizing turbine power sources the engines at idle provide substantial thrust. Accordingly, it is not unusual to find such turbine driven craft with steady-state taxi speeds in excess of safe speeds unless the brakes are employed continuously during taxiing. Of course such continuous use of the aircraft brakes to control taxi speed is not only awkward, but results in wear of the brakes and degradation of the braking performance if the brakes are overheated during taxi.
A particularly useful STOL aircraft concept is taught in U.S. Pat. No. 3,807,663 issued Apr. 30, 1974. In this patent, airfoil effectiveness and thrust is obtained by blowing an airstream over the airfoil from a spanwise plenum through a rearward facing slot. This airstream is preferably the bypass air from a turbofan engine. Though this concept is particularly effective in enhancing lift and, accordingly, low-speed take-off and flight, the above mentioned idle thrust of the engine continues to augment lift and thus decrease braking efficiency during the landing roll. The cumulative effect of this idle thrust is to produce higher than desirable taxi speeds and reduced braking effectiveness.
Another limitation upon low speed operation of STOL aircraft is the diminished effectiveness of conventional control surfaces at low airspeeds and the accompanying relatively high angle of attack of airfoil.